Field of the Invention
The present invention concerns a method to acquire, with a magnetic resonance apparatus, magnetic resonance data of a target region including a metal object, wherein a magnetic resonance sequence including at least one radio-frequency excitation to be emitted via a radio-frequency coil arrangement is used. The invention also concerns a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Magnetic resonance imaging is already widely known in the prior art. Radio-frequency excitations are thereby used in order to deflect spins in a basic magnetic field (B0 field) in order to be able to measure resulting signals from these. The radio-frequency field of the radio-frequency excitation is thereby most often designated as a B1 field.
Problems occur in magnetic resonance examinations when metallic objects are present, for example metallic implants in patients. In spite of these complications, the examination of patients with metallic implants has become an important application of magnetic resonance imaging. In particular, the increasing number of patients with orthopedic implants (for example screws, fixations, artificial joints, etc.) has led to the development of new techniques that are designed to reduce the significant image distortions due to the metal, since the high soft tissue contrast achieved with magnetic resonance imaging is superior to other imaging modalities. It should also be taken into account that other imaging modalities (for example computed tomography) also exhibit severe metal artifacts.
Magnetic resonance imaging is particularly appropriate to assist in treating post-operative complications since infections, rejection reactions and/or bone fractures can be markedly better diagnosed using magnetic resonance imaging, compared to other imaging modalities.
In magnetic resonance imaging, when a metallic object is present in the target region, the image artifacts are predominantly caused by the distortion or disturbance of the static basic magnetic field (B0 field), which is due to the high difference of the magnetic susceptibility of body tissue and metal. Additional basic known effects are the interference with dynamic magnetic fields (known as imaging gradients), for example due to eddy currents, as well as (as has recently been recognized) disruptions of the radio-frequency field due to induced radio-frequency currents in the metal object and in the surrounding tissue, for example.
Known procedures that target the reduction or correction of artifacts (in particular image distortions and contrast changes) due to metal objects have previously been implemented predominantly to address the severe distortions of the static B0 field near the metal objects. For example, it is known to use turbo spin echo sequences (TSE sequences) with a high bandwidth as the magnetic resonance sequence. Other approaches use a technique known as “view angle tilting” (distortion correction in the direction of the readout gradient). The distortion correction in the direction of the slice selection gradient—which is known under the keywords SEMAC/MAVRIC, see also in this regard the article by B. A. Hargreaves et al., “Metal-Induced Artifacts in MRI”, AJR: 197, 2011, P. 547-555—is additionally known.
Although effects with regard to other sources—the gradient-induced and radio frequency-induced eddy currents as well as radio-frequency shadowing effects—are known in the prior art, the proposed solutions are less practical for clinical use. Reference is made in this regard to an article by C. R. Camacho et al., “Nonsusceptibility Artifacts Due to Metallic Objects in MR Imaging”, JMRI: 5, 1995, P. 75-88. To solve this problem, it has been proposed to repeat the measurements multiple times with different transmitter voltages. However, this requires an unacceptable lengthening of the measurement times for the medical region. Alternatively, it has been proposed to use adiabatic pulses, but this requires long pulse lengths and high amplitudes, and therefore can only be used to a very limited extent.